In the field of medical engineering, many devices are known with which fluids can be withdrawn from a patient or delivered to a patient via a tubular conduit. The access to the patient is usually made with a catheter for insertion into organs of the body, or a cannula for puncturing vessels. During the examination or treatment, a correct access to the patient has to be ensured. It is therefore necessary to monitor the patient access.
In blood purification methods such as hemodialysis, hemofiltration and hemodiafiltration, blood is passed through an extracorporeal blood circuit. If the venous connection to the patient comes loose during the blood treatment, bleeding to death can be avoided only if the extracorporeal blood flow is stopped within a few seconds. Therefore, extracorporeal blood circuits are generally provided with protective systems which, in the event of an alarm, stop the blood pump, close the venous clamp and trigger an acoustic or optical warning signal.
DE 197 39 099 C1 describes a device for monitoring an access during an extracorporeal blood treatment, in which an electric current is induced in the connection of the extracorporeal blood circuit representing a closed conductor loop, the current flowing in the conductor loop is measured, and a characteristic change in the current strength points to an incorrect vascular access. In addition to inductive injection and output, it is also known to perform capacitive injection and output of electric signals in the extracorporeal blood circuit.
U.S. Pat. No. 6,932,786 B2 describes a monitoring device in which an AC voltage signal is capacitively injected and output in the extracorporeal blood circuit. The injection and output of the AC voltage signal takes place by means of electrical contact elements that enclose the tubular conduits. The electrical contact element in this case represents one “electrode” of a “capacitor”, while the blood flowing in the tubular conduits represents the other “electrode” of the “capacitor”. The insulating tubular conduit represents the dielectric of the capacitor lying between the electrodes.
In the known monitoring device, the AC voltage signal generated by an AC voltage signal generator is coupled to a venous contact element on the venous blood conduit and to an arterial contact element on the arterial blood conduit as a difference signal. In an alternative embodiment, one output of the frequency generator is connected to a contact element enclosing the venous blood conduit, while the other output of the signal generator is at ground potential. Both embodiments are based on the fact that the AC voltage signal is output as a difference signal with two contact elements that are arranged at different locations of the extracorporeal circuit, and the blood flowing in the extracorporeal circuit is at ground potential.
It has been found in tests that, in the method known from U.S. Pat. No. 6,932,786 B2, the output AC voltage signal can be superposed by relatively strong interference signals. In practice, therefore, the known device can prove relatively susceptible to faults.
US 2003/0195454 A1 deals with the problem of capacitive injection and output of measurement signals in the extracorporeal blood circuit and proposes injection and output of the measurement signals by means of electrical contact elements that are directly in contact with the blood flowing through the tubular conduits.
U.S. Pat. No. 7,060,047 describes a device for monitoring a vascular access during a dialysis treatment, which permits capacitive injection of an AC voltage signal, wherein an electrical circuit is closed via a common ground. The device in principle permits connection of the patient to ground. However, the document states that such a coupling of the patient is not absolutely essential.